1. Field of the Invention
This invention is concerned with devices, such as mirrors and windows, having controllable reflectivity.
2. Description of the Related Art
Sunlight transmitted through windows in buildings and transportation vehicles can generate heat (via the greenhouse effect) that creates an uncomfortable environment and increases air conditioning requirements and costs. Current approaches to providing "smart windows" with adjustable transmission for use in various sunlight conditions involve the use of light absorbing materials. These approaches are only partially effective, since the window itself is heated and because these devices, such as electrochromic devices, are relatively expensive and exhibit limited durability and cycle life. Certain liquid crystal-based window systems switch between transmissive and opaque/scattering states, but these systems require substantial voltages to maintain the transparent state. There is an important need for an inexpensive, durable low voltage smart window with variable reflectivity. Reflecting the light, rather than absorbing it, is the most efficient means for avoiding inside heating. Devices for effectively controlling transmission of light are also needed for a variety of other applications, e.g., energy efficient dimmers for displays.
Bright light from headlamps on following vehicles reflected in automobile rear and side view mirrors is annoying to drivers and creates a safety hazard by impairing driver vision. Currently available automatically dimming mirrors rely on electrochromic reactions to produce electrolyte species that absorb light that would otherwise be reflected from a static mirror. Such devices do not provide close control over the amount of reflected light, and are expensive to fabricate since a very constant inter-electrode spacing is required to provide uniform dimming. Image sharpness is also reduced for electrochromic mirror devices since the reflected light must pass through the electrolyte (twice). There is an important need for an inexpensive adjustable mirror device that provides close control of reflected light with minimal image distortion.
In early attempts to exploit reversible electrodeposition of a metal for light modulation, the deposits obtained on transparent substrates presented a rough and black, gray, or sometimes colored appearance (typical of finely-divided metals) and exhibited poor reflectivity and high light absorbance, especially when thick. Such deposits have been investigated for display applications involving reflectance from the background, with white pigments often being added to improve contrast. Warszawski (U.S. Pat. No. 5,056,899), which is concerned with displays, teaches that reversible metal electrodeposition is most appropriate for display applications, since significant disadvantages for transmission devices were given (e.g., the possibility of metal deposition at the counter electrode). In general, the prior art literature teaches that an auxiliary counter electrode reaction is required for transmission-type devices to avoid metal electrodeposition at the counter electrode as metal electrodissolution occurs at the working electrode, which would produce no net change in transmission. Such teachings imply that the application of reversible metal deposition to smart windows must involve light absorption by the finely divided electrodeposited metal, which would result in heating of the device itself and thus the space inside. The low reflectance of this type of deposit would not be appropriate for adjustable mirror applications.
Electrolytes described in the early prior art literature contain auxiliary redox species (e.g., bromide, iodide, or chloride) that are oxidized (e.g., to bromine, iodine, or chlorine) at the counter electrode during metal deposition under the high drive voltages used. This introduces chemistry-related instabilities during long term operation and leads to deposit self erasure on open circuit via chemical dissolution of the metal deposit, e.g., 2Ag.sup.0 +Br.sub.2 --&gt;2AgBr. In most cases, this auxiliary redox process hinders metal deposition at the counter electrode during erasure, introducing a threshold voltage that is desirable for display applications. This auxiliary redox process may represent a significant side reaction even when metal electrodeposition/dissolution occurs at the counter electrode and a threshold voltage is not observed. See, e.g., Warszawski, columns 3-4 (when copper or nickel were present in the counter electrode paste) and Duchene et al., Electrolytic Display, IEEE Transactions on Electron Devices, Volume ED-26, Number 8, Pages 1243-1245 (August 1979); French Patent No. 2,504,290 (Oct. 22, 1982). High switching voltages of at least 1 V were used for all the electrodeposition devices which have been found in the patent and literature prior art.
A paper by Ziegler et al. (Electrochem. Soc. Proc. Vol. 93-26, p. 353, 1993) describes an investigation for display applications of the reversible electrodeposition of bismuth in aqueous solutions containing a large molar concentration ratio of halide anions to the trivalent bismuth ion. Halide anion oxidation served as the counter electrode reaction with the 1.5 V write voltage used.
The deposits obtained were dark in color and were shown to decrease the reflectance of the ITO surface. Subsequent reports by these authors (Electrochem. Soc. Proc. Vol. 94-31 (1994), p. 23; Solar Energy Mater. Solar Cells 39 (1995), p. 317) indicated that addition of copper ions to the electrolyte was necessary to attain complete deposit erasure. These authors also utilized a counter electrode reaction other than metal electrodeposition/dissolution, and also never obtained a mirror deposit. Thus, Ziegler et al. provide no teachings relevant to the effect of electrolyte composition on the deposition/dissolution rate and quality of mirror electrodeposits.
Warszawski teaches that the use of a grid counter electrode would give a less uniform deposit since deposition on the transparent working electrode is highly localized in the vicinity of the counter electrode grid lines (a consequence of the very thin film of gel electrolyte used). Warszawski also teaches the use of an aqueous gel electrolyte to minimize sensitivity to atmospheric contaminants and to avoid the necessity of having a leak tight seal. Such electrolytes, however, have much more limited temperature and voltage operating ranges compared with organic-based electrolytes with high boiling solvents.
One effort to improve the deposit quality of the electrolytic solution used in a reversible electrodeposition process, described in U.S. Pat. No. 5,764,401 to Udaka et al., requires the addition of organic additives to the solution. Unfortunately, such additives are typically destroyed during the electrodeposition process, greatly limiting cycle life. Furthermore, this approach fails to produce highly-reflective mirror-like deposits that are required for adjustable mirror applications and provide the superior heat rejection needed for smart windows.
U.S. Pat. No. 5,880,872 to Udaka teaches that the "working" electrode of a reversible electrodeposition structure is degraded, and its working life thereby shortened, by the high voltage required to dissolve the metal film deposited upon it. Udaka states that this consequence can be avoided by adding an alkali metal halide to the device's electrolytic solution, preferably in an amount which provides an alkali metal halide to silver halide ratio of between 0.5 to 5. However, the described electrolytic formulation fails to provide the inherent stability, high quality deposits, good erasure and long cycle life needed for practical applications. Mirror deposits were never obtained.
Prior art literature teaches that the memory effect is temporary. This is a consequence of the occurrence of a counter electrode reaction other than metal electrodeposition/dissolution. The energetic oxidation products generated at the counter electrode can cause dissolution of the metal deposit on the working electrode either chemically on open circuit (slow) or electrochemically during short circuit (fast).
Nishikitani et al. (European Patent No. 0,618,477) teaches that the counter electrode in electrochromic devices for smart window applications can be a metal grid which is substantially transparent. Since no metal electrodeposition occurs in electrochromic devices, however, the grid in this case is used to provide a transparent electrode, not to maintain transparency by localizing metal deposition. In addition, to provide adequate electrical capacity for electrochromic devices, Nishikitani's grid would need a very high surface area (at least 10 m.sup.2 /g and preferably 50 to 5,000 m.sup.2 /g) and a line width of 50 to 5,000 .mu.m; alternatively, a plurality of dots on a conducting substrate can be used, but the dots must contain fine particles having electrical capacitance of not less than 1 farad/g.
A reversible electrochemical mirror (REM) device permitting efficient and precise control over the reflection/transmission of visible light and other electromagnetic radiation is described in U.S. Pat. Nos. 5,903,382 and 5,923,456 to Tench et al., which are assigned to the same assignee as the present application. In this device, an electrolyte containing ions of an electrodepositable metal is sandwiched between a mirror electrode and a counter electrode, at least one of which is substantially transparent to the radiation. A typical transparent mirror electrode is indium tin oxide (ITO) or fluorine doped tin oxide (FTO) deposited on a transparent glass (or plastic) pane which serves as the substrate. Application of a voltage causes the electrodepositable metal, e.g., silver, to be deposited as a mirror on the mirror electrode while an equal amount of the same metal is dissolved from the counter electrode. When the voltage polarity is switched, the overall process is reversed so that the mirror metal is at least partially dissolved from the mirror electrode. A thin layer of noble metal, e.g., 15-30 .ANG. platinum, on the transparent conductor is usually required to improve nucleation so that a mirror deposit is obtained. The thickness of mirror metal layer present on the mirror electrode determines the reflectance of the device for radiation, which can be varied over a wide range.
The REM technology can be used to provide control of either light reflectance, transmission, or both. A transmissive REM device suitable for smart window applications utilizes a noble metal counter electrode that is locally distributed, e.g., in a grid, on a transparent substrate, e.g., glass, so that mirror metal deposited thereon does not appreciably increase light blockage. In this case, high light transmission is provided by a locally distributed counter electrode of relatively small cross-sectional area and the device reflectance/transmission is adjusted via the thickness of mirror metal on the mirror electrode. As described in U.S. Pat. No. 5,903,382 to Tench et al., such a transmissive counter electrode is not required for reflective REM devices used for adjustable mirror applications. An electrolytic solution providing the inherent stability, high deposit quality, complete deposit erasure, long cycle life and fast switching needed for most practical applications is described in U.S. patent application Ser. No. 09/356,730, filed Jul. 19, 1999, now U.S. Pat. No. 6,111,685 which is assigned to the same assignee as the present invention.
A significant problem with adjustable mirrors of the type suitable for automotive applications, including both REM and electrochromic mirrors, is that simple means for monitoring the reflectance of such devices are not available. Consequently, it is necessary to place a light sensor in front of the mirror to provide feedback so that the reflectance can be adjusted to the desired level. Such sensors are not only expensive but are also aesthetically undesirable, increase the bulkiness of the device, and typically monitor only a small area while blocking a portion of the mirror itself. Similar difficulties exist for variable transmission devices as well. An inherent means for monitoring the mirror state of adjustable reflectance/transmission devices could provide significant advantages in terms of costs, performance, space utilization and market acceptance.